There is no trait more silently costly than a closed mind - it doesn't even notice all the chances passing by, nor can it estimate their value. Considering how big and extreme the world is, this subjects one to unlimited opportunity cost. Consequently, few other flaws could be of higher priority to fix.
Stoicism holds that emotionality is a sign of immaturity and weakness, that it interferes with clear thinking and discipline. But I believe they distrust emotional decision-making because they are unskilled at it, rather than because it is useless. Perhaps the oh-so-mighty Stoics may even fear their emotions. I don't find feeling to indicate weakness, I think feeling can be very difficult, but also rewarding and eye-opening.
If there is one thing that life has repeatedly drilled into my head it is that none of us are as great as we think. But the ego avoids being humbled so fiercely - as if it were death itself - even though it could alternatively be perceived as an extraordinary gift - which doesn't have to feel embarrassing but could instead be enjoyed, and that to react that way may incentivize more growth.
There are few limits to how horrible life can become, and no boundaries people will not exploit (even to their own detriment). We are all born into a losing battle that only a fool could be hopeful about. And yet, if you are unreasonably optimistic anyways, even against seemingly impossible odds, no one can crush your spirit, or know for certain what incredible anomalies may lay ahead.